The present invention relates to an optical system for an observing equipment such as a binocular or a terrestrial telescope that has an image vibration compensation system.
Recently, binoculars provided with image vibration compensation systems for preventing vibration of observed image due to hand-held shaking of a user have been developed. Japanese Laid Open Publication No. HEI 6-43365 discloses an image vibration compensation system of a binocular that employs variable angle prism for each of telescopic optical systems of the binocular. FIG. 14 shows one example of the telescopic optical system that has the same arrangement of the optical components as the publication. The optical system includes an objective lens 1 that is a doublet, a variable angle prism 2 as a compensation element, an erecting system 3 and an eyepiece 4. When the optical system vibrates due to hand-held shaking of a user, the vertex angle of the variable angle prism 2 is controlled to stabilize the image.
In such a construction, however, since the variable angle prism 2 is located in the convergent light, if the vertex angle of the prism is changed, coma occurs. FIG. 15A shows the axial coma when the vertex angle of the variable angle prism 2 has not yet changed. FIG. 15B shows the axial coma when the variable angle prism 2 is changed in order to compensate the inclination of the optical system which was inclined by 1 degree. As shown in FIGS. 15A and 15B, the coma has been increased significantly when the vertex angle has been changed. Accordingly, the image viewed by a user is deteriorated due to the coma when the vertex angle of the variable angle prism 2 has been changed to compensate the vibration of the image.
Another example of the image vibration compensation system for binoculars is disclosed in Japanese Laid Open Publication No. HEI 6-308431. The binocular in the publication employs a compensation device that is attached in front of the objective lenses of the binocular. Since the objective lens of the telescopic optical system has the largest diameter in the system, and the device is arranged in front of the objective lenses, the entire system becomes large in size.